1. Field of the Invention
The present invention relates to methods of estimating the lifetime of a transistor degraded by hot carriers in an MOSFET and, more particularly, to a method of estimating the lifetime of a floating SOI-MOSFET.
2. Description of the Background Arts
In a bulk MOSFET, the lifetime of the transistor degraded by hot carriers can be estimated by measuring a substrate current Isub.
FIG. 21 schematically shows a cross sectional structure of a typical bulk N-channel MOSFET. In the MOSFET shown in FIG. 21, a source 2 and a drain 3 are formed at one main surface of a semiconductor substrate 1. A gate electrode 5 is formed between source 2 and drain 3 on substrate 1 with a gate insulating film 4 interposed therebetween. Substrate 1 and source 2 are connected to ground, and a drain voltage Vd is applied to drain 3 and a gate voltage Vg is applied to gate electrode 5.
In such bulk MOSFET, flow of electrons "e" through the channel region from source 2 to drain 3 may cause impact ionization as indicated by a star near drain 3. Impact ionization produces pairs of hole "h" and electron "e" which is the hot carrier with high energy. Holes "h" diffuse in substrate 1 and leave substrate 1 as substrate current Isub through a substrate electrode (or a well electrode). Substrate current Isub can be measured with an ammeter A as shown in FIG. 21.
Meanwhile, electrons "e" or hot carriers penetrate into gate insulating film 4, where they may be trapped or produce an interface state between the channel region and the gate insulating film. Such hot carriers degrade the transistor characteristics.
As can be seen from the foregoing, there is a close correlation between the substrate current and hot carriers since the substrate current is generated by hot carriers produced due to impact ionization in the vicinity of the drain. Therefore, the lifetime of a transistor degraded by hot carriers can be estimated by measuring the substrate current.
FIG. 22 schematically shows a cross sectional structure of a typical SOI-MOSFET. In this SOI-MOSFET, a buried insulating layer 6 and a semiconductor layer 1A are stacked on semiconductor substrate 1 in this order. In the SOI-MOSFET, semiconductor layer 1A is isolated from substrate 1 by buried insulating layer 6 and semiconductor layer 1A corresponds to substrate 1 in the bulk MOSFET shown in FIG. 21. Source 2 and drain 3 are formed in semiconductor layer 1A. The region of semiconductor layer 1A excluding source 2 and drain 3 is called a body region 1B. Between source 2 and drain 3, gate electrode 5 is formed on semiconductor layer 1A with gate insulating film 4 interposed therebetween. Source 2 is connected to ground, drain voltage Vd is applied to drain 3 and gate voltage Vg is applied to gate electrode 5.
As apparent from FIG. 22, generally there is no such thing as a substrate electrode in an SOI-MOSFET. Therefore, in general the lifetime of a transistor degraded by hot carriers cannot be estimated from the substrate current.
It is also understood from FIG. 22 that holes "h" produced by impact ionization tend to accumulate in body region 1B, since body region 1B is isolated from substrate 1 by buried insulating layer 6, thereby increasing the potential of the body region. Increase in potential in body region 1B turns on an NPN bipolar transistor formed by source 2, body region 1B and drain 3, thereby further increasing the current between source 2 and drain 3 (parasitic bipolar effect). The current increased by such parasitic bipolar effect leads to an increase in production of hot carriers to accelerate degradation in transistor characteristics. Thus, such parasitic bipolar effect also makes it difficult to estimate the lifetime of an SOI-MOSFET degraded by hot carriers.
FIG. 23 is a graph showing the results obtained by measuring the lifetime of the SOI-MOSFET as shown in FIG. 22. In this graph, the reciprocal 1/Vd(1/V) of the drain current is represented on the horizontal axis and the lifetime .tau.(sec) of the transistor is represented on a log scale on the vertical axis. The measured SOI-MOSFET has a channel length L of 0.3 .mu.m and gate voltage Vg of 0.7V is applied thereto as the hot carrier stress condition. As can be seen from FIG. 23, the lifetime of the transistor sharply declines in the region with greater drain voltage Vd (the left region of the graph). Thus, since plotting of the transistor lifetime does not result in a straight line, accurate estimation of the lifetime of the SOI-MOSFET is difficult. For example, drain voltage Vd allowed for the SOI-MOSFET to have the lifetime of ten years is 3.13V or lower if estimated from the two leftmost measurement points in the graph, but is 1.96V or lower if estimated from the four measurement points on the right side of the graph. Although plotting of the transistor lifetime presents in good linearity in the region with a relatively small drain voltage, measurement of the transistor lifetime in such region requires a long stress test of approximately 10.sup.4 -10.sup.6 seconds, preventing achievement of a simple lifetime estimation method.